1. Field of the Invention
The present invention generally relates to measurement of the cost of providing electric service, and more particularly, relates to accurate measurement of power consumption related quantities.
2. Related Art
The ultimate goal in the art of metering is to precisely identify the cost of providing electric service. Early meters have undergone many changes and improvements over time, all in an effort to improve further the accuracy of measurements involved in determining such cost.
Although electric utility systems are designed based on total kilovolt-amperes (kVA) required by a load to be served, real power consumed over time, or energy, is the quantity typically measured for billing a consumer. Kilovolt-amperes is referred to as apparent power. Apparent power can be visualized as being composed of two components---kilowatts (kW) and kilovars (kVAR). Kilowatts sometimes is referred to as "real power" and kilovars sometimes is referred to as "reactive power".
To better understand the difference between real power and reactive power, consider an induction motor (a typical load in an electric utility system). In order to operate the motor, two current components may be considered-magnetizing (reactive) current and power-producing (real) current. The magnetizing current is the current required to produce the magnetic fields necessary for the operation of the motor. Without magnetizing current, energy would not flow through the core of a transformer or across an air gap. The product of magnetizing current and voltage is reactive power. The power-producing current is the current which is converted into useful work performed by the motor. The product of power-producing current and voltage is real power.
Although both real and reactive power are required from an electric utility system, it was recognized as early as 1892 that real power by itself reasonably represents the cost of providing electric service. Since measurement of the real power is a long-accepted and reliable quantity in determining the cost of providing electric service, any additional improvements to determining such cost should not only be significant, but also cost effective. Before now, the conflicting goals of keeping meter costs down but also providing a meaningful reactive power measurement prevented widespread use, especially for residential services, of reactive power measurements.
With regard to the cost of measuring reactive power, and referring again to the induction motor example, an induction device typically introduces both an inductive (magnetizing) reactance and a resistance (real) into an electric utility system. In the presence of only a resistive load, current flowing in the system is in-phase with the voltage, i.e., no magnetizing current is required. When an inductive load is introduced into the system, however, total current flowing in the system is shifted out-of-phase with the voltage by an angle depending upon the relative amounts of resistance and reactance. Although total current flowing in the system is shifted out-of-phase with the voltage, the total current may be visualized as being composed of two components--an in-phase component and an out-of-phase, or quadrature, component.
Reactive power can be measured by a technique commonly known as phase-shifting. The phase-shifting technique requires that a meter be configured so that the applied voltage, i.e., a voltage of a magnitude representative of the magnitude of the line voltage, in the meter be displaced 90 degrees (lagging for inductive reactance) from the phase angle of the line voltage. The applied voltage, therefore, is substantially in-phase (at least in a "vector" sense) with the quadrature current component. The product of the phase-shifted applied voltage and the current thus is a measure of reactive power.
Although known phase-shifting techniques for determining reactive power are technically feasible, the known techniques have many economic disadvantages. Particularly, a reactive power measure by itself is not an accepted measure of the cost of providing electric service. A real power measure must also be provided.
Until recently, one meter could not be configured to measure both real and reactive power. Therefore, two separate meters typically are utilized--one meter to measure the reactive power and another meter to measure the real power. The added costs associated with utilizing two meters to make such measurements is highly undesirable.
Even if reactive power can be measured in some cost-effective manner, a problem hindering the cost-effective utilization of reactive power measurements is the relationship between volt-amperes (apparent power), watts (real power), and reactive volt-amperes (reactive power). More specifically, once reactive and real power measurements are obtained, these measurements must be combined in some meaningful way as a measure of electrical energy consumption. In linear sinusoidal circuits, a well-known relationship between apparent, real and reactive power is: EQU kVA=[kW.sup.2 +kVAR.sup.2 ].sup.1/2 ( 1)
where:
kVA=kilovolts-amperes (apparent power); PA1 kW=kilowatts (real power); and PA1 kVAR=reactive kilovolt-amperes (reactive power).
To utilize Equation (1), a relatively sophisticated processor, compared to the processors utilized in most meters, is required. Particularly, meters typically utilize 8-bit digital processors with limited functionality and limited random-access memory. To minimize hardware costs, the processors preferably only are required to perform add/subtract and multiply/divide functions rather than the more complicated, and more computationally expensive, square/square root functions. To carry out the operations recited in Equation (1), however, a processor must be able to perform square/square root operations. The processing costs associated with carrying out Equation (1) therefore are high.
With known meter devices, in an attempt to avoid increasing the cost of a processor utilized in the reactive power meter, and instead of processing data "on-site" where it is gathered, the data is stored in a local memory device (sometimes referred to herein as a recorder) of the meter. More particularly, a pulse initiating device coupled to the meter generates a discrete pulse whenever a predetermined quantity of reactive power has been measured. Each pulse stored in the recorder therefore represents a predetermined quantum of reactive energy. The pulses are stored (e.g., on magnetic tape, solid state memory device, etc.) along with time interval pulses. A similar pulse initiating device and memory device are utilized with the real power (watthour) meter.
Once a month or so, a meter reader (typically a human) gathers the stored pulse data, e.g., a memory "dump" from the recorder memory to an electronic meter reader memory, and/or to a remote memory read by telephone, radio, or other means. The gathered pulse data is then provided to a central processing system. At the central processing site, the apparent power (kVA) for each respective time interval is determined from the real and reactive power pulses from each respective time interval. Apparent power demand (kVA demand), for example, is then determined using the following relationship: EQU kVA demand=[kVAh consumed in a timed interval]/ (2) EQU [time duration of the interval]
A maximum kVA demand from a single time period is then identified. The central processor also sums the kWH pulses to obtain the total kWH energy supplied.
The utilities bill consumers for total kilowatt-hour, i.e., real power consumed during the billing period. The maximum kVA demand is used to bill a consumer for investment related costs, e.g., the cost of equipment required to furnish the consumer with electricity. Particularly, in addition to recovering for the cost of real energy consumed, a utility needs to recover the capital cost associated with the supply/distribution system. A reasonable way to recover such cost is to charge users according to the user's maximum current requirement or the user's maximum apparent power requirement. A user with a high maximum current requirement requires more capital investment by the utility (e.g., a larger transformer and lines) than a low maximum current requirement user.
Additionally, most users have lagging VAR loads due to the predominance of inductive devices such as motors, ballasts, transformers and the like connected to the system. To compensate for the inductive loads, the utility must operate its generators at a leading phase angle. Operating generators at other than zero phase angle reduces the capacity of the generators to generate real power for transmission to the load. The maximum kVA demand inherently includes a measure of the amount of leading phase angle required to compensate for each consumer's load.
With the above described and well-known system, in addition to the expense of using two meters, the meters must be equipped with respective pulse initiating devices and memory/storage devices. Further, the kVAR meter must be equipped with a phase shifting transformer. A sophisticated central data processing system operated by skilled workers also is required. The equipment cost of such a system inhibits its wide-spread application. The system typically is only used for measuring energy requirements of large revenue users.
Additionally, with the above-described system, a consumer cannot observe the status of energy consumption during a billing period and the system also does not provide real-time apparent power quantities. The above-described system provides only time-averaged quantities at the end of the billing period. Real-time information is useful, for example, so that variations in power factor can be determined during each time interval. If the power factor becomes too small (e.g., if the reactive power is large as compared to the real power) during a particular time period, the consumer may want to take steps to reduce reactive power requirements. In this manner, a consumer can attempt to minimize maximum kVA demand.
In spite of the importance of reactive power measurement, reactive power often is not measured--especially in single phase residential services. Costs associated with measuring reactive power using known systems are high. As previously explained, however, a poor ratio of kW to kVA, i.e., low power factor, has a serious effect on the economic design and operating costs of a system. When power factor is low and rates are based only on kilowatt-hours, the utility is not being compensated for the power (kVA) required to generate, transmit, and distribute.
Until now, no known meter system provides both an economically obtained measure of reactive power and an economic manner of using the reactive power measure to achieve the ultimate metering goal, i.e., accurate measurement of the cost of providing electric service. Further, no known meter system provides a relatively low cost simple manner of determining, in real-time, both real and reactive power requirements and power factor.